1. Field of the Invention
This invention relates to an exhaust gas purification device provided in an exhaust passage of an internal combustion engine to purify NOx contained in exhaust gas.
2. Description of the Related Art
In order to improve fuel consumption characteristics and exhaust gas characteristics, a lean burn engine such as an in-cylinder injection type engine is operated at a lean air-fuel ratio, i.e., an air-fuel ratio leaner than the stoichiometric air-fuel ratio in a specified operational region.
In such lean burn engine, while lean air/fuel ratio operation (hereinafter referred to as “lean operation”) is being performed, NOx (nitrogen oxides) contained in exhaust gas is not purified in a sufficient manner only with a three-way catalyst. Hence, by providing a NOx catalyst, the emission of NOx into the atmosphere is reduced. The NOx catalyst absorbs NOx contained in exhaust gas in lean operation. The NOx catalyst discharges the absorbed NOx and purifies the NOx by reducing the NOx with a reducing agent (NOx purging) in stoichiometric air-fuel ratio operation (hereinafter referred to as “stoichiometric operation”) or rich air-fuel ratio operation (hereinafter referred to as “rich operation”).
When the three-way catalyst and the NOx catalyst are provided, in order to ensure a sufficient exhaust gas purification performance immediately after the internal combustion engine is started, the three-way catalyst is arranged in an upstream section of an exhaust passage of the internal combustion engine to become activated in early time.
When the three-way catalyst is arranged upstream of the NOx catalyst, however, a reducing agent such as HC or CO is purified by the three-way catalyst also during NOx purging, which causes a problem that NOx purging is not performed in a sufficient manner.
Thus, in order to perform NOx purging in a sufficient manner even when the three-way catalyst is arranged upstream of the NOx catalyst, techniques such as making the rich operation time longer, making the air-fuel ratio for the rich operation further richer, or injecting additional fuel after combustion of main fuel have been developed and proposed, for example, by Japanese Unexamined Patent Publication No. 2002-4915 (hereinafter referred to as “patent document 1”).
However, making the rich operation time longer, or making the air-fuel ratio for the rich operation further richer as in the technique disclosed in patent document 1 causes a problem that the fuel economy of the internal combustion engine deteriorates to a great degree.
Further, the purification capacity of a catalyst changes depending on temperature, and there is a problem that as the three-way catalyst becomes more activated, the amount of reducing agents supplied to the NOx catalyst during NOx purging decreases, so that the exhaust gas purification performance of the NOx catalyst lowers.